Current operational trends are towards minimising the size and quantity of fluid conduits necessary to operate a subsea well, particularly as operations in deeper water are becoming more commonplace. One of the emerging intervention techniques involves the introduction of tooling into the well via a subsea intervention system. The intervention system comprises a safety package or lower riser package secured to the Christmas tree, a lubricator extending upwards from the lower riser package and a pressure control head, which is mountable to an open end of the lubricator. In this system there is no riser to surface, the intervention tooling being deployed through open water, transported with the pressure control head down to the lubricator. The pressure control head is landed on a latch at the open end of the lubricator, which seals and secures the pressure control head in position.
The intervention system must be able to manage three potentially conflicting operational requirements. The first is it must be robust enough to prevent the escape of high-pressure hydrocarbon well fluids in order to prevent potential damage to personnel, equipment and the environment. The second requirement is the system needs to be open mouthed in order to allow entry of wireline tooling and the pressure control head. The third requirement is that in order to avoid the creation of hydrates, which may occur under certain pressure and temperature conditions, the system must prevent mixing of gaseous high-pressure hydrocarbon well fluids with seawater, which occurs during every entry and departure of wireline tooling. To prevent the hydrate formation, seawater or hydrocarbons are displaced from the lubricator as necessary.
The displacement of seawater is generally achieved by pumping a flushing agent, such as monoethylene glycol (MEG), from a surface vessel down to the lower riser package and lubricator. MEG is a suitable flushing agent because it does not form hydrates when mixed with hydrocarbon gases and is also denser than seawater. When the MEG is pumped into the lower riser package and lubricator the MEG sinks to the bottom of the lower riser package and push the sea water upwards, out of the lubricator.
When downhole intervention operations take place, the intervention system becomes an extension of the well and the intervention system fills with hydrocarbon gas and/or oil. At the end of intervention operations, prior to opening the intervention system and recovery of the intervention tooling, the hydrocarbon must be removed from the intervention system. Conventional systems deal with this task by flushing the intervention system to, for example, (i) vent the hydrocarbons out into the sea (with the associated risk of forming hydrates and causing pollution), (ii) recover the hydrocarbons to surface for storage or (iii) force the fluids back into the well.
The nature of the intervention operations and the number of trips in and out of the well with tooling may require a number of exchanges between well fluids and MEG.
Whilst this system works reasonably successfully for shallow wells, in deep-sea environments the system encounters drawbacks. For example to pump the MEG from a surface vessel requires a longer, heavier hose, which adds to the cost of producing from deep sea wells and the vessels themselves require special equipment for handling the MEG and, in some cases, for controlling the heavier hose. Furthermore, if the hydrocarbon gas is recovered to surface, the vessel to which the hydrocarbon is recovered requires specialist equipment and personnel for dealing with this material. Alternatively, if the hydrocarbons are released into the sea, significant environmental damage can occur.